Respiratory humidification system

ABSTRACT

A flow probe for use in a humidification system is disclosed. The flow probe is adapted to be positioned in a humidified gases flow (for example oxygen or anaesthetic gases) such as that which is provided to a patient in a hospital environment. The flow probe is designed to provide both temperature and flow rate sensing of the gases flow by incorporating two sensors (preferably thermistors) and the shape and alignment of the probe enables accurate readings by reducing the occurrence of condensation on the sensors. A number of possible applications are disclosed wherein the flow sensor is included in humidification control systems which provide a patient with a desired humidity level or simplify the amount of user input required or wherein the flow sensor provides a controller with flow information which may then be used to determine certain, possibly dangerous, conditions (such as incorrect flow sensor placement, breathing circuit disconnected, no water in the humidification chamber or humidity out of required limits).

This is a divisional of U.S. patent application Ser. No. 09/585,867,filed Jun. 1, 2000, which is a divisional of U.S. patent applicationSer. No. 09/097,832, filed or Jun. 16, 1998 which issued as U.S. Pat.No. 6,349,722 on Feb. 26, 2002.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to gases distribution systems and in particular,though not solely, to respiratory humidifier systems which humidifygases for a patient, or other person in need of such gases, to breathe.

2. Description of the Prior Art

Many, if not all, existing respiratory humidification systems whichdeliver humidified gases (such as oxygen or anaesthetic gases) to apatient, or other person in need of such gases, operate as temperaturecontrollers. That is, the temperature of the gases leaving thehumidification device in the breathing circuit is monitored and the heatsource controlled in response to changes in that temperature to achievea desired outgoing humidified gases temperature. An example of this typeof humidifier control system is disclosed in our prior U.S. Pat. No.5,558,084. This method of control has a number of disadvantagesincluding:

In situations with high incoming gases temperature (approaching thedesired outgoing gases temperature) little heat is necessarily suppliedto the gases by the humidification process to achieve the requiredtemperature. Accordingly, little humidification of the gases is alsoachieved.

The dependency on temperature sensors in this control method means thatincorrect placement or connection of the temperature sensors can lead toimpaired performance of the entire humidification and breathing system.

Lack of flow sensors which, if provided, would enable certain breathingcircuit conditions to be easily recognised and appropriate action to betaken by the humidification device (and/or the gases supply). Flowsensors have previously not been utilised in humidification systems dueto insufficient robustness and problems of condensation occurring on theflow sensor, leading to incorrect flow readings.

Gases being supplied to the patient at a pressure/humidity combinationwhich is inappropriate. It is well known that certain humidity levelsare required of gases which are to be administered to a patient.Different humidity values are specifically suitable to intact (forexample face mask) or bypassed (intubation delivery of gases) airways.Temperature sensing alone can not ensure that these requiredtemperature/humidity values are achieved.

Some existing respiratory humidification devices require users to adjustdials which have little or no intuitive relationship to the actualphysical parameters they are intended to control. Often the dials adjustthe required gases outlet temperature and/or the heating supplied by theheater wire provided within the conduit connecting humidifier to patient(and sometimes also the conduit connecting the patient back to the gasessupply). The most important parameter in humidified gases supply to apatient is the humidity of the gases as insufficient humidity can veryquickly dehydrate the patient's airways. Accordingly, users have littleor no idea where to position the dials to produce the desired result ofoptimum humidity in the supplied gases at the existing flow rate. Anautomated system in which the user need only inform the humidificationdevice if the patient receiving the humidified gases has intact orby-passed airways would be a major advance.

Many existing respiratory humidification devices display the gasestemperature being supplied to the patient. As previously mentioned, themost important parameter in respiratory humidification systems is thehumidity of the gases. Often, the temperature displayed has norelationship to the actual humidity of the gases being supplied to thepatient due to heating in the delivery circuit and can therefore bemisleading to the average health care professional. It would, therefore,be an advantage if the temperature displayed was in some way related toor indicative of the humidity of the gases being supplied to thepatient.

BRIEF SUMMARY OF THE INVENTION

It is, therefore, an object of the present invention to provide arespiratory humidifier system which will go at least some way towardsovercoming the above disadvantages or which at least provide theindustry with a useful choice.

Accordingly, in a first aspect, the invention consists in sensor probemeans adapted for positioning in a flow of humidified gases comprising:

sensor housing means adapted for positioning in said gases flow, saidsensor housing means having a longitudinal axis substantiallyperpendicular to said humidified gases flow and a sensing end,

sensing means housed within said sensor housing means at or near saidsensing end, and

at least one projecting tab means extending laterally from said sensorhousing means, said at least one projecting tab means providing surfaceswhich enable liquid condensate to disperse away from said sensing end ofsaid sensor housing means.

In a second aspect, the invention consists in humidification apparatusfor humidifying a gases flow to be supplied to a patient or other personin need of such gases comprising:

humidification chamber means adapted to hold a quantity of water andhaving an inlet and an outlet to allow said gases flow to pass throughsaid humidification chamber means,

heating means provided adjacent said humidification chamber means andadapted to provide heat to said quantity of water in said humidificationchamber means in order to provide water vapour to said gases flowpassing through said humidification chamber means,

gases transportation pathway means connected to said outlet of saidhumidification chamber means to convey said gases flow to said patientor other person in need of such gases,

flow probe means adapted to sense the flow rate of said gases flow,

temperature sensing means adapted to sense the temperature of said gasesflow,

user input means to allow a user to set a required temperature of saidgases flow,

control means which receives input from said flow probe means, saidtemperature sensing means and said user input means and controls saidheating means in response to said inputs to maintain said gases flow atsaid required temperature.

In a third aspect, the invention consists in humidification apparatusfor humidifying a gases flow to be supplied to a patient or other personin need of such gases comprising:

humidification chamber means adapted to hold a quantity of water andhaving an inlet and an outlet to allow said gases flow to pass throughsaid humidification chamber means,

heating means provided adjacent said humidification chamber means andadapted to provide heat to said quantity of water in said humidificationchamber means in order to provide water vapour to said gases flowpassing through said humidification chamber means, said heating meansutilising a measurable quantity of power,

gases transportation pathway means connected to said outlet of saidhumidification chamber means to convey said gases flow to said patientor other person in need of such gases,

flow probe means adapted to sense the flow rate of said gases flow,

control means which receives input from said flow probe means andstoring a program which causes the control means to:

i) calculate the power usage required of said heating means in order tohumidify said gases flow to a predetermined level at the gases flow ratesensed by said flow probe means,

ii) determine the power presently being utilised by said heating means,and

iii) supply said predetermined level of power to said heating means ifthe determined present power utilisation of said heating means is lessthan said required power usage.

In a fourth aspect, the invention consists in humidification apparatusfor humidifying a cases flow to be supplied to a patient or other personin need of such gases comprising:

humidification chamber means adapted to hold a quantity of water andhaving an inlet and an outlet to allow said gases flow to pass throughsaid humidification chamber means,

heating means provided adjacent said humidification chamber means andadapted to provide heat to said quantity of water in said humidificationchamber means in order to provide water vapour to said gases flowpassing through said humidification chamber means,

gases transportation pathway means connected to said outlet of saidhumidification chamber means to convey said gases flow to said patientor other person in need of such gases,

flow probe means adapted to sense the flow rate of said gases flow,

temperature sensing means adapted to sense the temperature of said gasesflow,

user input means to allow a user to select a desired gases humiditylevel of said gases flow,

data storage means containing information on target gases temperaturesat various gases flow rates for a number of humidification chamberoutlet means temperatures,

control means which receives input from said temperature sensing meansand said user input means and using said flow information from said flowprobe means repeatedly obtains corresponding target temperatureinformation from said data storage means corresponding to the desiredgases humidity level and varies the heat provided by said heating meansuntil the sensed temperature is substantially equivalent to said targettemperature in order to obtain said desired gases humidity level.

In a fifth aspect, the invention consists in humidification apparatusfor humidifying a gases flow to be supplied to a patient or other personin need of such gases comprising:

humidification chamber means adapted to hold a quantity of water andhaving an inlet and an outlet to allow said gases flow to pass throughsaid humidification chamber means,

heating means provided adjacent said humidification chamber means andadapted to provide heat to said quantity of water in said humidificationchamber means in order to provide water vapour to said gases flowpassing through said humidification chamber means,

gases transportation pathway means connected to said outlet of saidhumidification chamber means to convey said gases flow to said patientor other person in need of such gases,

flow probe means adapted to sense the flow rate of said gases flow,

temperature sensing means adapted to sense the temperature of said gasesflow,

user input means which may be in one of a predetermined number of statescorresponding to one of a number of gases delivery configurations, eachconfiguration optimally requiring a predetermined gases temperature andhumidity level,

control means which receives input from said flow probe means, saidtemperature sensing means and said user input means and controls saidheating means to provide said gases flow to said patient or other personin need of such gases at a temperature and humidity level as indicatedby said user input means.

In a sixth aspect, the invention consists in humidification apparatusfor humidifying a gases flow to be supplied to a patient or other personin need of such gases comprising:

humidification chamber means adapted to hold a quantity of water andhaving an inlet and an outlet to allow said gases flow to pass throughsaid humidification chamber means,

heating means provided adjacent said humidification chamber means andadapted to provide heat to said quantity of water in said humidificationchamber means in order to provide water vapour to said gases flowpassing through said humidification chamber means,

gases transportation pathway means connected to said outlet of saidhumidification chamber means to convey said gases flow to said patientor other person in need of such gases, said gases transportation pathwaymeans having a patient end, distal to said end connected to said outletof said humidification chamber means,

first temperature sensing means adapted to sense the temperature of saidgases flow substantially at said outlet of said humidification chambermeans,

second temperature sensing means adapted to sense the temperature ofsaid gases flow substantially at said patient end of said gasestransportation pathway means,

display means adapted to display temperature information to a user,

control means which receives input from said first temperature sensingmeans and outputs a signal to said display means to cause a temperatureto be displayed to the user which is the lower of the temperaturessensed by said first and said second temperature sensing means.

In a seventh aspect, the invention consists in humidification apparatusfor humidifying a gases flow to be supplied to a patient or other personin need of such gases comprising:

humidification chamber means adapted to hold a quantity of water andhaving an inlet and an outlet to allow said gases flow to pass throughsaid humidification chamber means,

heating means provided adjacent said humidification chamber means andadapted to provide heat to said quantity of water in said humidificationchamber means in order to provide water vapour to said gases flowpassing through said humidification chamber means,

gases transportation pathway means connected to said outlet of saidhumidification chamber means to convey said gases flow to said patientor other person in need of such gases,

flow probe means adapted to sense the flow rate of said gases flow, and

control means which receives input from said flow probe means andcompares the sensed flow rate of said gases flow with a predeterminedminimum required gases flow rate and places the humidification apparatusinto a safe mode of operation if the sensed rate is less than saidpredetermined minimum rate.

In an eighth aspect the invention consists in humidification apparatusfor humidifying a gases flow to be supplied to a patient or other personin need of such gases comprising:

humidification chamber means adapted to hold a quantity of water andhaving an inlet and an outlet to allow said gases flow to pass throughsaid humidification chamber means,

heating means provided adjacent said humidification chamber means andadapted to provide heat to said quantity of water in said humidificationchamber means in order to provide water vapour to said gases flowpassing through said humidification chamber means,

gases transportation pathway means connected to said outlet of saidhumidification chamber means to convey said gases flow to said patientor other person in need of such gases, said gases transportation pathwaymeans having a patient end, distal to said end connected to said outletof said humidification chamber means,

humidity sensing means which senses the humidity of said gases flowbeing supplied to said patient,

timer means which may be used to time certain humidification apparatusevents,

alarm means which may be energised to provide a warning signal after apredetermined alarm time,

storage means which stores said alarm times for a number of associatedsensed humidity values, and

control means which stores a program which causes the control means to:

i) receive input of said sensed humidity value from said humiditysensing means,

ii) obtain from said storage means the alarm time associated with saidsensed humidity value,

iii) start said timer means,

iv) wait until the time elapsed by said timer means substantially equalssaid alarm time and

v) energise said alarm means to provide said warning signal.

In a ninth aspect the invention consists in humidification apparatus forhumidifying a gases flow to be supplied to a patient or other person inneed of such gases comprising:

humidification chamber means adapted to hold a quantity of water andhaving an inlet and an outlet to allow said gases flow to pass throughsaid humidification chamber means,

heating means provided adjacent said humidification chamber means andadapted to provide heat to said quantity of water in said humidificationchamber means in order to provide water vapour to said gases flowpassing through said humidification chamber means,

heating means power utilisation sensing means which monitors the levelof power being used by said heating means,

heating means temperature sensing means which senses the temperature ofsaid heating means,

gases transportation pathway means connected to said outlet of saidhumidification chamber means to convey said gases flow to said patientor other person in need of such gases, said gases transportation pathwaymeans having a patient end, distal to said end connected to said outletof said humidification chamber means,

alarm means which may be energised to provide a warning signal after apredetermined alarm time, and

control means which stores a program which causes the control means to:

i) determine a difference temperature by subtracting the gasestemperature determined by said gases flow temperature sensing means fromthe heating means temperature sensed by said heating means temperaturesensing means,

ii) determine a power requirement value for the heating means from saidheating means power utilisation sensing means,

iii) calculate a thermal conductivity value by dividing said powerrequirement value by said difference temperature,

iv) energise said alarm means if said calculated thermal conductivityvalue is less than a predetermined minimum allowable thermalconductivity value.

In a tenth aspect the invention consists in humidification apparatus forhumidifying a gases flow to be supplied to a patient or other person inneed of such gases comprising:

humidification chamber means adapted to hold a quantity of water andhaving an inlet and an outlet to allow said gases flow to pass throughsaid humidification chamber means,

heating means provided adjacent said humidification chamber means andadapted to provide heat to said quantity of water in said humidificationchamber means in order to provide water vapour to said gases flowpassing through said humidification chamber means, said heating meansutilising a measurable quantity of power,

gases transportation pathway means connected to said outlet of saidhumidification chamber means to convey said gases flow to said patientor other person in need of such gases,

gases transportation pathway heating means which are energisable tosupply heat to said gases flow along at least a part of the length ofsaid gases transportation pathway means,

gases temperature sensing means which senses the temperature of saidgases flow leaving said humidification chamber means,

user input means to allow a user to enter a required temperature offsetvalue which is the required difference in temperature between the saidsensed gases temperature and the temperature of the gases flow deliveredto said patient,

control means which stores a program which causes the control means to:

i) determine the gases temperature of the gases leaving saidhumidification chamber means using said gases temperature sensing means,

ii) receive said offset temperature value from said user input means,

iii) calculate an airway set-point temperature by adding said gasestemperature to said offset temperature,

iv) energise said gases transportation pathway heating means to increasethe temperature of said gases flow by said offset value along the lengthof said gases transportation pathway means.

To those skilled in the art to which the invention relates, many changesin construction and widely differing embodiments and applications of theinvention will suggest themselves without departing from the scope ofthe invention as defined in the appended claims. The disclosures and thedescriptions herein are purely illustrative and are not intended to bein any sense limiting.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention consists in the foregoing and also envisages constructionsof which the following gives examples only.

One preferred form of the present invention will now be described withreference to the accompanying drawings in which;

FIG. 1 is a front elevation of a flow probe constructed according to onepreferred embodiment of the present invention,

FIG. 2 is a view from below of the flow probe of FIG. 1,

FIG. 3 is a cross-sectional side elevation of a breathing circuitshowing the flow probe of FIG. 1 installed within the conduit,

FIG. 4 is a cross-sectional view from below of the breathing circuit ofFIG. 3 showing the flow probe of FIG. 1 installed in the conduit,

FIG. 5 is a schematic diagram of a respiratory humidification systemincorporating the flow probe of FIG. 1,

FIG. 6 is a flow diagram of one preferred embodiment of a humidity andtemperature control system utilised in the respiratory humidificationsystem shown in FIG. 5,

FIG. 7 is a graph of the target outlet temperature required (for adesired humidity level) versus flow rate illustrating one preferredembodiment of a humidity and/or temperature control system utilised inthe respiratory humidification system shown in FIG. 5, and

FIG. 8 is a graph of humidity (or dewpoint) versus time to alarm in anexample humidification system such as that shown in FIG. 5.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

With reference to the accompanying drawings and in particular to FIG. 5,an example humidification apparatus or respiratory humidification systemincorporating preferred embodiments of the present invention isillustrated. Included in the respiratory humidification system is aventilator or gases supply means or blower 1 having an outlet 2 whichsupplies gases (for example oxygen, anaesthetic gases or air) to theinlet 3 of a humidification chamber means 4 via a conduit 6.Humidification chamber means 4 may, for example comprise a plasticsformed chamber having a metal base 7 sealed thereto. Humidificationchamber 4 is adapted to hold a volume of water 8 which is heated by aheater plate means 9 under the control of controller or control means 11of a humidification device or humidifier 10.

As the water within chamber 4 is heated it will slowly evaporate, mixingwater vapour with the gases flow through the humidification chamber fromventilator 1. Accordingly, humidified gases leave humidification chamber4 via outlet 12 and are passed to a patient or other person in need ofsuch gases 13 through a gases transportation pathway or inspiratoryconduit 14. In order to reduce condensation within the inspiratoryconduit 14 and to raise the temperature of the gases provided to thepatient 13 a heating wire means 15 may be provided which may beenergised under the control of control means 11.

In FIG. 1 a gases mask 16 is shown over the patient's nose and mouth(referred to as “Intact Airways” gases delivery) however it should beunderstood that many gases delivery configurations exist such asintubation in which a delivery tube is positioned in the patient'strachea to by-pass the patient's airways (known as “Intubated Airways”gases delivery). It is also possible to provide a return path for thepatient's exhaled gases back to ventilator 1. In this case a suitablefitting such as a “Y-piece” may be attached between the patient (13),inspiratory conduit (14) and an expiratory conduit (not shown) which isconnected to an inlet (not shown) of ventilator 1.

Control means 11 may for example comprise a microprocessor or logiccircuit with associated memory or storage means which holds a softwareprogram which, when executed by control means 11, controls the operationof the humidification system in accordance with instructions set in thesoftware and also in response to external inputs. For example, controlmeans 11 may be provided with input from heater plate 9 so that controlmeans 11 is provided with information on the temperature and/or powerusage of the heater plate 9. In addition, control means 11 could beprovided with inputs of temperature of the gases flow, for example atemperature sensing means or temperature probe 17 may be provided at ornear the patient to indicate the gases temperature being received by thepatient and a further temperature probe 18 may be provided to indicateto control means 11 the temperature of the humidified gases flow as itleaves outlet 12 of humidification chamber 4. Furthermore, a flowsensing means or flow probe 19 may be provided anywhere in the breathingcircuit (“the breathing circuit” comprises the parts of thehumidification apparatus through which the gases flow passes). The flowprobe 19 is shown in FIG. 5 in the same position as temperature probe 18as the two devices may both be provided in one probe as will describedbelow.

A still further input to control means 11 may be a user input means orswitch 20 which could be used to allow a user (such as a health careprofessional or the patient themselves) to set a desired gasestemperature of gases to be delivered or a desired gases humidity levelto be delivered or alternatively other functions could be controlled byswitch 20 such as control of the heating delivered by heater wire 15 orselecting from a number of automatic gases delivery configurations(which will be described below).

A number of preferred embodiments of the system (or parts thereof) setout above will now be described in more detail.

Flow Probe

With reference to FIGS. 1 and 2, the preferred form of flow probe 19 isshown. Flow probe 19 is preferably formed by moulding in a plasticsmaterial such as polycarbonate and comprises a base portion 30 adaptedto hold wire conductors (48 in FIGS. 3 and 4) which carry electricalsignals to and from control means 11. Protruding from base 30 is a stem31 which has at least one sensor housing means 32 and 33 protruding fromits end furthest from base 30. Sensor housing means 32 and 33 arepreferably rounded in cross-section and substantially tapered or conicalin elevation with a rounded tip at the end (the sensing end 36) farthestfrom base 30.

Shown in FIG. 1 are two sensor housing means 32 and 33. In theembodiment shown, one sensor housing means 32 is provided as atemperature sensing means while the other sensor housing means isprovided to perform the function of flow rate sensing means. Withinsensor housing means 32 and 33 are sensing means 34 and 35, for examplethermistors (temperature dependent resistors), which are provided tosense the respective properties of temperature and flow rate of thegases flowing in the humidification system. In the case of thetemperature sensing means 34, controller 11 may provide a voltage acrossthe thermistor and receive a temperature signal in the form of thecurrent passing through the thermistor which will be dependent on thetemperature of the gases. To protect thermistor 34, sensor housing means32 completely encases or encapsulates the thermistor, however as thereis only a thin layer of plastics material between the thermistor and thegases flow, the temperature reading obtained is still accurate.

In the case of the flow sensing means 35, controller 11 may on occasionprovide a current to the thermistor for a duration sufficient to warmthe thermistor to a first known temperature and then disconnect thecurrent supply and monitor the change in temperature of the thermistor(by monitoring its change in resistance). Controller 11 may then start atiming means and determine the length of time taken for the thermistor'stemperature to drop to a second predetermined temperature. The timetaken for the thermistor 35 to change in temperature form the first tothe second known temperature along with a known cross-sectional area ofgases flow (for example a 12 mm diameter conduit) providing controller11 with an indication of the flow rate of the gases as they conduct heataway from the heated thermistor. It can be seen that thermistor 35 isnot encased or encapsulated in the same way as thermistor 34. This isbecause any layer of material between the thermistor 35 and the gasesflow would influence the heat transfer rate from thermistor to gases andthus reduce the accuracy of the flow rate reading.

In a more preferable embodiment, the flow rate of she gases flow wouldbe determined by supplying current to thermistor 35 to raise itstemperature above the temperature of the gases flow by a preselecteddifference temperature, for example 60° C. Controller 11 then monitorsthe power drawn by thermistor 35 in maintaining the fixed temperaturedifference. The power usage in association with the cross-sectional areaof the gases flow (for example a 12 mm diameter conduit in the region ofthe flow probe) provide the controller 11 with an indication of the flowrate, allowing the controller to determine the actual flow rate of thegases. In order for thermistor 35 to maintain the difference temperatureit will be necessary to occasionally determine the actual temperature ofthermistor 35 while also heating thermistor 35. This may be achieved byremoving the heating current from the thermistor temporarily and placinga low sensing voltage across thermistor 35 and sensing the currentthrough thermistor 35. In this way the resistance of thermistor 35 canquickly be measured and a value of temperature deduced from previouslystored characteristic temperature versus resistance data for thermistor35. The sensing voltage may then be removed and the heating currentreapplied if the predetermined temperature difference has not beenachieved or controller 11 may delay applying further heating tothermistor 35 if the temperature difference has been met or exceeded.

As the exposed surfaces of flow probe 19 will generally be at a lowertemperature than the humidified gases flow passing over it, condensationis likely to occur on its surfaces. It should be understood that anyliquid water accumulating on the flow sensing thermistor 35 willadversely affect the flow rate reading as the liquid water will absorbsome of the heat produced by the thermistor. In order to reduce oreliminate the occurrence of liquid water accumulation on the sensors,the flow probe according to the preferred embodiment of the presentinvention is provided with at least one “wing” or projecting tab meansand in the example shown in FIGS. 1 and 2 two tab means (37, 38, 39 and40) are shown per sensor housing means (although it may be possible toutilise one projecting tab means per sensor housing means). Incross-section each tab means is preferably rectangular and extends alongthe length of the sensor housing means from stem 31 to the sensing endof the sensor housing means (although it may not be necessary for theprojecting tab means to extend the full length of the sensor housingmeans). In the preferred embodiment the outer edge of the projecting tabmeans is substantially a constant distance from the centre line of thesensor housing means along its entire length. As the sensor housingmeans is tapered, the projecting tab means is therefore triangular inelevation extending preferably perpendicularly from the sensor housingmeans surface. Preferably the projecting tab means are integrallymoulded with the flow probe 19, however, it would be possible toseparately manufacture the projecting tab means and attach them to thesurface of the sensor housing means.

With reference now also to FIGS. 3 and 4, in use, flow probe 19 isinserted into a sensor entry port 41 in a conduit connector 42. Sensorentry port 41 comprises a substantially cylindrical wall extendingperpendicularly from conduit connector 42. Conduit connector 42 connectstwo conduits 43 and 44 of the breathing circuit or may alternatively bemoulded as part of a conduit, for example, as part of inspiratoryconduit 14. As may be seen most clearly in FIG. 4, the flow probe 19 ispositioned with relation to the gases flow (indicated by arrows) toensure that the projecting tab means 37, 38, 39 and 40 are each alignedparallel to the gases flow. As condensation forms on the sensor housingmeans it is caused to run away from the sensor end 36 by the action ofthe gases flow passing over its surface combined with a localised regionof low surface tension in the vicinity of the line of contact of theprojecting tab means and the surface of the sensor housing means.Accordingly, condensate tends to flow along the line of intersection(for example line 45) away from sensor end 36 towards stem 31 asdesired.

In order to ensure that, upon insertion of flow probe 19 into sensorentry port 41, the projecting tab means are correctly aligned with thegases flow (as incorrect alignment will not produce the desired affectof removing liquid from the sensor tip), the preferred embodiment of thepresent invention also includes a substantially “V” shaped locatingtooth means 46 adjacent the stem 31 and also projecting from baseportion 30. A complimentary substantially “V” shaped notch or fixedlocating depression 47 is provided in the wall of sensor entry port 41.Accordingly, a user inserting the flow probe 19 will find that in orderto fully and securely insert the flow probe into the conduit (or conduitconnector), it will be necessary to rotate the flow probe until thelocating tooth means 46 and locating depression 47 are combined at whichtime the flow probe will be correctly aligned to ensure thatcondensation will tend to run away from the sensor tips as previouslydescribed.

Furthermore, in order to ensure that heat generated by the operation ofthe flow sensing thermistor 35 does not substantially impact upon thetemperature sensing thermistor 34, it can be seen in FIG. 4 that uponalignment of locating tooth means 46 and locating depression 47, thetemperature and flow sensing thermistors 34, 35 are displaced across thegases flow (that is, they are not aligned in the direction of flow) sothat they are each substantially unaffected by the others presence.Also, the heat producing flow sensing thermistor 35 is positioneddownstream of the temperature sensing thermistor 34 so that thegenerated heat is carried away from the temperature sensing thermistor34 by the gases flow.

An advantage of providing a reliable flow probe in the humidificationapparatus according to the preferred form of the present invention isthat the humidification apparatus can recognise conditions which wouldimpair the humidification apparatus' performance (such as occurrences ofsuctioning, circuit disconnects and nebulising treatments) by monitoringthe flow rate and or temperature for telltale indicative conditions.Once it is determined that a certain recognised condition is occurring,appropriate action may be taken (such as raising an alarm or removingheat from heater plate 9). The humidification apparatus could, forexample, determine if the temperature probes have been incorrectlyplaced or removed from the circuit by, for example sensing no flow withan associated low (ambient) temperature.

The following are a number of preferred uses or applications for theflow probe according to the preferred form of the present invention.

Humidifier Control System—Minimum Power Method

An important parameter of the gases flow supplied to the patient 13 orother person in need of such gases is the humidity. It is well knownthat gases which are too dry (having a low relative humidity of betweenaround 60% and 70%) can very quickly dehydrate the patient's airwayscausing discomfort. The controller 11 of the humidification apparatusaccording to the preferred embodiment of the present inventionpreferably includes a control system which attempts to maintain therelative humidity of the gases flow at a desirable level (greater thanabout 90%). One situation where this type of control is desirable iswhere the temperature of the inlet gases to the humidification chamber 4rises to a temperature similar to the gases outlet temperature. In thissituation, as very little energy is required to be supplied to the gases(to raise their temperature), it is not possible to provide sufficientenergy to the water 8 in the humidification chamber and thereforeinsufficient water vapour is available to humidify to the gases,accordingly, while the temperature of gases supplied to the patient 13is desirable, the relative humidity is not. When the incoming gasestemperature is much less than the required outlet gases temperature thenit can virtually be assumed that in the process of providing a largeamount of energy in raising the gases temperature to the required value,much water will have been vaporised in the humidification chamber 4 andaccordingly the relative humidity of the gases will be high.

In order to control the humidity of the gases flow reaching the patient,the humidification apparatus according to the present invention requiresinformation relating to the flow rate of the gases. This may be achievedby inserting a flow probe, preferably as described above, into the gasesflow. This control system will now be described with reference to theflow diagram of FIG. 6.

The control system starts at block 49 with heater plate 9 beingenergised to provide heat to the water within the humidification chamber4. At block 50 controller 11 reads a required humidity which has eitherbeen preset in memory by the manufacturer or has been input by a uservia a user input such as user input 20 in FIG. 5. At block 51 controller11 receives information from flow sensing thermistor 35 in order todetermine the flow rate of the gases flow (this may be accomplished aspreviously described). At block 52 controller 11 determines the minimumpower required to generate the required humidity level in the gases flowat the sensed flow rate. This may be achieved by carrying out acalculation using a formula stored in memory or, preferably, a datastorage means or memory device associated with the control means 11 hasa data look up table of flow rates and their associated minimum requiredpower values at a number of desired humidity levels stored therein whichis interrogated by the control means using the sensed flow rate and therequired humidity value. Control means 11 could determine the requiredpower level of heater plate 9 by sensing the gases flow rate andreceiving a user input desired humidity level and calculating (oralternatively obtaining from a look up table of experimentally derivedor previously calculated values) a required evaporation rate to obtainthe desired humidity level at that flow rate. Controller 11 could thencalculate (or alternatively obtain from a look up table ofexperimentally derived or previously calculated values) the powerrequired to be supplied by heater plate 9 in order to produce thedetermined evaporation rate thus ensuring the required humidity level isachieved.

At block 53 (which is not an essential step in the method) the controlmeans 11 controls the temperature of the gases leaving the outlet of thehumidification chamber at a preset (either by the user or manufacturer)temperature (for example 37° C.) in the known way by varying the heaterplate 9 temperature or power with gases outlet temperature feedbacksupplied to the controller via temperature sensor 18 (or by thetemperature sensing part of flow probe 19).

At block 54, the present power utilisation of the heater plate 9 isdetermined and a decision is made as to whether the present powerutilisation of the heater plate is less than the value calculated atblock 52. The present power utilisation could, for example be calculatedby the controller 11 sensing the current supplied to the heater plateand multiplying this sensed current value by the voltage supplied to theheater plate. Alternatively, the heater plate average power could bedetermined by calculating the percentage of time for which the heaterplate is energised and multiplying this by the rated power value of theheater plate. For example, if the heater plate is energised for 40% ofthe time and the rated power of the heater plate is 150 Watts then theaverage power utilised by the heater plate would be 60 Watts. It couldbe assumed that the heater plate voltage will be constant. If thepresently determined power utilisation is not less than the minimumvalue determined to be necessary to provide the desired humidity levelthen control returns to block 50 where the previously described stepsare repeated, the patient receiving appropriately humidified gases,until the decision at block 54 reveals that the heater plate powerconsumption has dropped below the required level to supply adequatelyhumidified gases.

At this point, control passes to block 55 where the power supplied toheater plate 9 is increased (for example by varying a pulse widthmodulated supply voltage to the heater plate or simply increasing avariable voltage supply) to the level determined in block 52 in order toensure that the gases are adequately humidified. This will cause theoutlet gases temperature to rise above the set temperature, however thisis necessary in order to provide adequate humidity. A check is then madeat block 56 (which is also not a required step in the method) to see ifthe outlet gases temperature has dropped below a predeterminedtemperature (say 37° C.). If the outlet gases temperature has droppedbelow the predetermined temperature then it can be assumed that thegases will be receiving the required level of humidity as they are at atemperature sufficiently above the assumed gases inlet temperature. Ifthe outlet gases temperature has not dropped to below the predeterminedtemperature then the calculated minimum power level continues to besupplied to the gases. It can therefore be seen that:

1) in the absence of a temperature sensor, the control system willcontinually supply to the heater plate the calculated minimum requiredpower to achieve adequate humidification, or

2) where a temperature sensor is supplied, the control system willoperate in two modes, a first “normal” mode where the outlet temperatureis controlled in the known way to a desired temperature until the powerutilisation of the heater plate drops to a level which indicatesinsufficient humidification at which point a new control mode operatesto maintain the heater plate power usage at the calculated minimum leveluntil the outlet gases temperature drops below a preset temperatureindicating that the inlet gases temperature has dropped sufficiently toallow the humidification chamber to supply sufficient heat and humidityto the gases flow.

Humidifier Control System—Desired Humidity Method

An alternative humidifier control system to that set out above will nowbe described with reference to FIG. 7. According to this alternativepreferred control system, it is possible to control the humidity of thegases leaving the humidification chamber 4 to any desired level at anygases flow rate. This is made possible by determining the gases flowrate, preferably using the flow probe described above, along withknowledge of the humidification chamber output versus flow and orbreathing circuit characteristics.

An example of the humidification chamber output characteristics areshown in FIG. 7 where it can be seen that for a given required gaseshumidity level, as gases flow rate is increased, the temperature of thegases at the humidification chamber outlet drops rather steeply and thensettles to a substantially constant temperature. This information may beexperimentally derived for a number of target gases outlet temperaturesand humidity levels and recorded in a memory storage device (for examplein the form of a look-up table or a number of look-up tables) searchableby control means 11.

In accordance with this control system, the user enters a desiredhumidity level to controller 11 by way of a user input device such asuser input means 20 which may in this case comprise a dial or electronickey pad. The heater plate 9 is then energised to warm the water withinhumidification chamber 4 and temperature probe 18 (or the temperaturesensing part of flow probe 19) is used to provide a sensed outlet gasestemperature signal to control means 11. Utilising the present flow ratevalue sensed by flow probe 19 and the sensed temperature, controller 11interrogates its memory device to determine the target outlet gasestemperature required to achieve the desired humidity level at thepresent gases flow rate.

At this point control means 11 controls the energisation of heater plate9 in order to obtain the determined target outlet gases temperaturewhich will provide the required level of humidity at the present gasesflow rate. Energisation of heater plate 9 may, for example, take theform of pulse width modulation of the voltage supply to vary the powersupplied to the heater plate or alternatively a variable voltage supplycould supply the heater plate.

As changes are made in either the flow rate of the gases or in the userset desired humidity level, controller 11 automatically determines anupdated target outlet gases temperature from its storage device andappropriately controls heater plate 9 to provide that outlet gasestemperature.

For example, for a user set desired humidity level of 44 mg H₂O perliter of gases and a sensed flow rate F₁, controller 11 will interrogatethe tables in the storage device to determine a target gases outlettemperature of 37° C. is required. Controller 11 then energises heaterplate 9 in such a way (for example by PWM control of the supply voltageor current) that the outlet gases temperature sensed by temperaturesensor 34 is substantially equal to the target temperature of 37° C.resulting in the desired absolute humidity of 44 mg H₂O per liter.

As an addition to this control system, the memory device associated withcontrol means 11 could also be supplied with information relating to thecondensation characteristics of the inspiratory conduit. A heater wire15 may be energised by control means 11 to control the additionalheating to the gases as they pass along the conduit to thereby reducecondensation in the conduit. This also reduces changes in the humiditylevel of the gases along the conduit (as less water will come out of thegases as condensation). In this control system controller 11 may adjustthe heating supplied by heater wire 15 so that as well as controllinghumidity of the gases flow the temperature may also be controlled(although in practice the heater wire could only supply a few degrees oftemperature increase). However, controller 11 may also conceivably usethe heater wire setting to reduce humidity of the gases if they werebeing supplied at an excess level (in order to produce gases of asufficiently high temperature) by causing rain-out to occur. Controlmeans 11 would then manipulate the heater plate and heater wire settingsappropriately to provide the required gases humidity and temperature(set by a user) to the patient to the best of its ability.

Automated Humidification Apparatus—“Single Button Humidifier”

As a result of implementing either of the above control systems in thehumidification apparatus of FIG. 5, it would be possible to provide ahumidifier which was extremely simple to use, requiring minimal inputfrom a user. An example of a simple to use humidification apparatuswould be as shown in FIG. 5 with the only user input being switch 20.Switch 20 would preferably have a number of states or positionscorresponding to a predetermined number of gases deliveryconfigurations. One gases delivery configuration could be IntubatedAirways and another could be Intact Airways. For each position or stateof switch 20, a corresponding optimally required humidity value andtemperature value is stored in a memory associated with controller 11.For example, for the Intubated Airways configuration the optimaltemperature may be about 37° C. and the optimal humidity value about 44mg H₂O per liter of gases while the Intact Airways optimal temperaturemay be about 32° C. and the optimal humidity value about 30 mg H₂O perliter of gases.

By utilising either one of the above described control systems it wouldthus be possible to control operation of the humidification apparatuswithout further user intervention once the gases delivery configurationis known. The controller 11 would repeatedly sense outlet gasestemperature and flow rate and adjust heater plate power and possiblyheater wire setting to automatically provide optimal (or as near tooptimal as possible) gases temperature and humidity to patient 13,independent of changes in flow rate or inlet gases temperature.

User Output—Temperature Display

A further feature of the humidification apparatus according to a furtheraspect of the present invention is the incorporation of a display means60 (FIG. 5) for displaying to the user the gases temperature beingsupplied to the patient 13. It should be noted that this feature doesnot rely on the presence of a flow probe in the breathing circuit.Display means 60 is controlled by control means 11. It is known thatother respiratory humidifiers incorporate display means, however, thetemperature which is displayed is invariably fixed at either thetemperature of the gases at the patient end of inspiratory conduit 14(as sensed by temperature sensor 17) or the temperature of the gases atthe humidification chamber outlet (as sensed by temperature sensor 18).

Many health care professionals equate the displayed temperature with theamount of moisture contained in the gases. So long as the gases suppliedto the patient are at 100% relative humidity (that is, the gases containas much water vapour as they can possibly hold at their presenttemperature) then the temperature of the gases supplied to the patientwould be clinically accurate. However, if the delivered gases containless than the maximum possible amount of moisture at their presenttemperature, then a humidifier which simply displays the delivered gasestemperature could mislead a health care professional into believing thatthe patient is receiving more humidity than they actually are.

In the preferred form of the present invention, the temperature which isdisplayed on display means 60 is either the temperature sensed by sensor14 or sensor 18, whichever is the lowest. As an example, a gases outlettemperature of 37° C. and an absolute humidity of 44 mg H₂O per liter ofgases (approximately 100% relative humidity) may translate to aninspiratory conduit patient end temperature of 35° C. and an absolutehumidity of 35 mg H₂O per liter of gases at the patient. Accordingly 9mg H₂O per liter of gases is condensing in the inspiratory conduit whilethe gas remains at approximately 100% relative humidity along theconduit due to the drop in temperature. In this situation, theappropriate temperature to display to the user is 35° C. as a gas at arelative humidity of 100% at this temperature contains the amount ofmoisture indicated by a temperature of 35° C.

If however the gases outlet temperature was 37° C. with an absolutehumidity of 44 mg H₂O per liter of gases (100% relative humidity) andthe patient end temperature was 39° C. with an absolute humidity of 44mg H₂O per liter of gases then the most clinically relevant temperatureto display would be 37° C. This is because the gases arriving at thepatient will no longer be at 100% relative humidity as no extra moisturehas been provided to the gases along the inspiratory conduit althoughthe gases have risen in temperature. The absolute humidity of the gasesarriving at the patient is actually associated with a gases temperatureof 37° C. as this is the temperature corresponding to the amount ofmoisture within the humidified gases. In any event, as the patient endtemperature is often measured at a distance of up to 30 cm from thepatient, by the time the gases arrive at the patient they have oftendropped and so the lower temperature of 37° C. is even more relevant tohealth care professionals.

Automatic No Flow Standby Mode

As has previously been mentioned, in many existing humidificationsystems, the controller simply senses temperature in order to adjustpower delivered by the humidifier heater plate 9 and/or conduit heaterwire 15. In a situation where the gases supply means or blower 1 isdisconnected from the breathing circuit these types of controllers willsense a lack of temperature as there will be no gases flow passing thetemperature sensor. The controller then attempts to increase thetemperature of the gases (which it assumes are still flowing in thebreathing circuit) by increasing the power supplied to heater plate 9and/or heater wire 15. As the temperature sensors are not able toregister any increase in temperature of the “flow”, the controller 11may continue to increase the power supplied to heating the non-existentgases flow to a dangerous level. If the gases supply is thenre-established, the gases supplied to the patient could be at an unsafetemperature.

In order to avoid the above series of events occurring, the flow sensoraccording to the preferred form of the present invention could beincorporated into a humidification system. The controller could thendetermine if the humidifier has sufficient gases flow (say, for example1.5 liters per minute) for normal safe operation. If the gases flow isfound to be insufficient then the humidifier could be placed into a safemode of operation. The safe mode could include a limit on heater plate 9temperature and/or limits on the duty cycle of voltage supplied toheater plate 9 and/or heater wire 15 (that is, control of power levels).

Humidity Alarm

It is believed that an alarm (such as an audible and/or visual alarm)should be provided in a humidification system to warn the patient (orhealth care professional) when the gases supplied to the patient havebeen below (or above) the required humidity level for a period of time.It has been found that the alarm should be set to go off after a periodof time which is dependent on the difference between the requiredhumidity and the actual humidity level being supplied to the patient.The larger the difference, the sooner the alarm should occur.

FIG. 8 shows one possible graphical example of how the time delay may beset, based on the patient's physiological humidity needs. A number ofdifferent such “humidity profiles” could be stored in a memory device,each one based around a predetermined required humidity value (theexample shows a required humidity value of 37° C.). The relationshipbetween temperature difference and time to alarm could conveniently beexpressed in a table format stored in, for example, ROM (Read OnlyMemory) to be read by control means 11 such that the control meansdetermines the humidity difference, looks up that difference in a table(the table selected depending on the required humidity value) whichprovides the appropriate time to wait before issuing the alarm. Analternative to measuring the humidity of the gases supplied is tomonitor the actual dew point (temperature at which condensation startsto occur) of the gases and to determine the difference between theactual dewpoint and the required or optimal dewpoint (for example 37°C.). The actual dew point could, for example be assumed to be the lowerof the humidification chamber 4 temperature and the conduit 14temperature.

Water Out Alarm

In a respiratory humidification system incorporating a humidificationchamber 4, it is imperative that a certain minimum level of water ismaintained in order for the humidifier to have the ability to supplywater vapour to the gases supply. Accordingly, the health careprofessional administering humidified gases to the patient shouldoccasionally check the water level and add more water when required.This job is sometimes overlooked.

It is possible to utilise flow probe 19 in a humidification system whichautomatically determines when the water level drops to an insufficientlevel and raises an alarm. Preferably, the heater plate 9 temperature,the humidification chamber 4 temperature (or chamber outlet temperature)and heater plate 9 power requirement (the amount of power presentlybeing supplied to the heater plate) are all monitored and utilised inthe following equation to provide a value for Thermal Conductivity:${ThermalConductivity} = \frac{HeaterPlatePowerRequirement}{{HeaterPlateTemperature} - {ChamberTemperature}}$

Controller 11 compares the calculated thermal conductivity value to apredetermined threshold value (which itself is dependent on the gasesflow rate determined by flow probe 19) which may be experimentallydetermined at various gases flow rates. The calculated ThermalConductivity value could for example, be updated every 5 minutes forexample and an alarm could, for example, be raised after a period of 5or 10 minutes have elapsed from the calculated Thermal Conductivityvalue dropping below the threshold (alternatively the alarm could beissued immediately). The following are experimentally determinedexamples of Thermal Conductivity values and preferred example thresholdvalues at different flow rates:

Flow rate=10 liters/minute

Thermal Conductivity=1.26 W/°C. (with sufficient water in chamber 4)

Thermal Conductivity=0.26 W/°C. (without water in chamber 4)

Predetermined threshold=0.5 W/°C.

Flow rate=40 liters/minute

Thermal Conductivity=1.81 W/°C. (with sufficient water in chamber 4)

Thermal Conductivity=0.42 W/°C. (without water in chamber 4)

Predetermined threshold=0.8 W/°C.

The predetermined threshold values at a number of flow rates could bestored in ROM accessible by controller 11 so that the controller wouldsimply determine the present flow rate of the gases, calculate the valueof Thermal Conductivity, access the table in ROM based on the presentflow rate and read out the associated predetermined threshold value. Ifthe calculated threshold value is greater than the calculated ThermalConductivity value then controller 11 would wait the predeterminedperiod of time (for example, 5 or 10 minutes) before issuing an alarm sothat the water level could then be topped up without a loss of humidityin the gases being delivered to the patient.

Chamber Set-Point Tracking

In a respiratory humidification system including a conduit heater wire,temperature and humidity are usually controlled so that gases suppliedto the patient arrive at required temperature and humidity levels. Insome situations the conduit heater wire 15 supplies sufficient energy toraise the temperature of the gases in the breathing circuit to achievethe desired temperature at the patient. On some occasions, the limitedpower available from the conduit heater wire (even at 100% duty cycle)is insufficient to raise the gases temperature to the requiredtemperature of gases for the patient. More particularly, the inabilityof these humidification systems to maintain the required gasestemperature at the patient end of conduit 14 usually results incondensation or “rain-out” occurring in the conduit due to thehumidified gases giving up too much of their heat to the conduit walls.The controller according to a further preferred embodiment of thepresent invention includes a system to minimise or alleviate the aboveproblem.

Accordingly, rather than attempting to maintain the patient gasestemperature at a desired level, the respiratory humidification systemaccording to a preferred form of the present invention attempts tomaintain a “temperature gradient” along the length of conduit 14 andadjusts the required patient temperature (or “Airway set-point”)accordingly. The Airway set-point value is calculated as follows:

Airway set-point=chamber outlet temperature+offset

Where the value of “offset” is for example 3° C. and equates to thedesired temperature gradient required along the conduit 14. It should benoted that the value of “offset” chosen is dependent on the physicalproperties and configuration of the conduit.

For example, for an offset of 3° C. and a humidification chamber 4outlet gases temperature of 37° C., the heater wire 15 will be energisedappropriately (for example by adjusting its duty cycle) to maintain thetemperature of gases supplied to the patient at 40° C. Similarly, if thechamber outlet temperature dropped to 31° C. then the temperature ofgases supplied to the patient would be controlled to arrive at 34° C. Inboth instances, a temperature gradient or difference of +3° C. ismaintained along the conduit minimising or eliminating condensation.

If it is found that the required offset value is not maintainable (thatis, the heater wire is incapable of raising the temperature of the gasesin the conduit to the calculated required value sensed by, for example atemperature sensor near the patient end of conduit 14) then controller11 will decrease the humidification chamber outlet temperature (by forexample, reducing the duty cycle of power supplied to heater plate 9) inorder to maintain the required offset temperature along the conduit. Asan example, the controller could be programmed to begin to drop thehumidification chamber outlet temperature in 0.5° C. steps (to a minimumof for example 35.5° C.) if the offset temperature value is notmaintainable at at least 2° C. for 15 minutes. For example, for anoffset value of 3° C. and an initial chamber outlet temperature of 37°C., the gases supplied to the patient should be controlled to arrive at40° C. If however the gases arriving at the patient are at 38.6° C. (anactual offset or difference of only 1.6° C.), then controller 11 willdrop the humidification chamber outlet temperature to 36.5° C. after 15minutes. The above calculations will then be repeated and if thetemperature of gases reaching the patient is not maintainable at 39.5°C. then controller 11 will again consider dropping the humidificationchamber temperature. This process will be repeated until ahumidification chamber outlet temperature is reached at which therequired conduit offset temperature can be maintained. Furthermore, thecontroller 11 could then attempt to raise the humidification chamberoutlet gases temperature so that the gases supplied to the patient canagain be established at a required temperature but only if this can beaccomplished under the offset temperature constraint. This would only bepossible if ambient circumstances had changed.

Thus, at least in the preferred form, the present inventionincorporating all or some of the above described features provides arespiratory humidification system which enables humidity and/ortemperature control of the humidified gases to be achieved. The gasesflow probed according to one embodiment of the present invention enablesaccurate flow rate measurements to be made without condensationaffecting the sensor. In part this increased accuracy is also due to thelocating system which ensures correct alignment of the flow and/ortemperature probe in the gases flow. Due to the ability to accuratelysense flow rate with this flow sensor, the control systems according tothe present invention are able to provide a gases flow to the patientwhich is controlled to a required humidity. The flow rate sensor alsoenables “automatic” control to be achieved whereby the user is notrequired to constantly monitor the output of the humidifier and to alterinputs to achieve desired changes, the user is merely required to informthe humidifier of the patient's gases delivery situation and thehumidifier is able to provide the required gases temperature andhumidity without further user input. The humidifier also displays agases temperature value which is clinically relevant to the gasesreaching the patient. In addition, the respiratory humidificationaccording to other preferred embodiments of the present inventionencompasses various safety improvements over the prior art.

What is claimed is:
 1. Humidification apparatus for humidifying a gasflow to be supplied to a patient or other person in need of such gascomprising: humidification chamber means for holding a quantity of waterand having an inlet and an outlet to allow said gas flow to pass throughsaid humidification chamber means, gas flow temperature sensing meansfor sensing the gas temperature within or adjacent said humidificationchamber means, heating means provided adjacent said humidificationchamber means for providing heat to said quantity of water in saidhumidification chamber means in order to provide water vapour to saidgas flow passing through said humidification chamber means, heatingmeans power utilisation sensing means for monitoring power being used bysaid heating means, temperature sensing means for sensing thetemperature of said heating means, gas transportation pathway meansconnected to said outlet of said humidification chamber means forconveying said gas flow to said patient or other person in need of suchgas, said gas transportation pathway means having a patient end, distalto said end connected to said outlet of said humidification chambermeans, alarm means for providing a warning signal after a predeterminedalarm time, and a controller connected to said gas flow temperaturesensing means, said heating means temperature sensing means and saidheating means power utilization sensing means, said controllerconfigured to: i) determine a difference temperature by subtracting thegas temperature determined by said gas flow temperature sensing meansfrom the temperature sensed by said temperature sensing means, ii)determine a power requirement value for the heating means from saidheating means power utilisation sensing means, iii) calculate a thermalconductivity value by dividing said power requirement value by saiddifference temperature, iv) activate said alarm means if said calculatedthermal conductivity value is less than a predetermined minimumallowable thermal conductivity value, wherein said controller providesthat said humidification apparatus is configured to sense thetemperature of the heating means, sense the gas flow temperature andsense the power used by said heating means and calculate thermalconductivity based on what is sensed, and output an alarm depending onthe calculated thermal conductivity.
 2. Humidification apparatus asclaimed in claim 1 which also comprises flow probe means for sensing theflow rate of said gas flow and storage means for storing a number ofsaid predetermined minimum allowable thermal conductivity values withassociated gas flow rates, and said controller is also configured tocarry out the step of iii a) determining the gas flow rate from saidflow probe means and obtaining from said storage means the predeterminedminimum allowable thermal conductivity value associated with thedetermined gas flow rate.
 3. Humidification apparatus as claimed inclaim 1 or claim 2 wherein said controller is also configured to carryout the step of v) waiting a predetermined period of time and thenrepeating steps (i) to (v).
 4. Humidification apparatus as claimed inclaim 2 wherein said flow probe means comprises: sensor housing meansfor positioning in said gas flow, said sensor housing means having alongitudinal axis substantially perpendicular to said humidified gasflow and a sensing end, sensing means for sensing said gas flow housedwithin said sensor housing means at or near said sensing end, and atleast one projecting tab means extending laterally from said sensorhousing means for providing surfaces which enable liquid condensate todisperse away from said sensing end of said sensor housing means. 5.Humidification apparatus as claimed in claim 4 wherein said flow probemeans comprise two or more said projecting tab means.
 6. Humidificationapparatus as claimed in claim 5 wherein said two projecting tab meansare oppositely positioned around said sensor housing means. 7.Humidification apparatuses claimed in claim 4 or claim 5 wherein eachsaid at least one projecting tab means is aligned parallel to said gasflow.
 8. Humidification apparatus as claimed in claim 4 or claim 5wherein liquid condensate is dispersed along the lines of intersectionbetween said sensor housing means and said at least one projecting tabmeans, there existing a localised area of low surface tension along saidlines of intersection.
 9. Humidification apparatuses claimed in claim 4or claim 8 wherein said sensor probe means comprise two sensor housingmeans being defined by a temperature sensor housing and a flow ratesensor housing.
 10. Humidification apparatus as claimed in claim 9wherein said sensing means of said temperature sensor housing and saidflow rate sensor housing each comprise a temperature dependentresistance.
 11. Humidification apparatus as claimed in claim 9 whereinsaid sensing means of said flow rate sensor housing is configured to beoccasionally heated to a predetermined difference temperature above thetemperature of said gas flow, and wherein the power required by saidsensor means of said flow rate sensor housing to maintain saidpredetermined difference temperature providing an indication of the flowrate of said gas.
 12. Humidification apparatus as claimed in claim 9wherein said sensing means of said flow rate sensor housing is exposedat or near the sensing end of the flow rate sensor housing while thesensing means of said temperature sensor housing is encapsulated at ornear the sensing end of the temperature sensor housing. 13.Humidification apparatus as claimed in claim 9 herein said temperaturesensor housing and flow rate sensor housing are spaced across said gasflow in order that heat produced by the sensing means of said flow ratesensor housing has substantially minimal effect on the sensing means ofsaid temperature sensor housing.
 14. Humidification apparatus as claimedin claim 11 wherein said flow rate sensor housing is positioneddownstream of said temperature sensor housing in order that heatproduced by the sensing means of said flow rate sensor housing does noteffect the sensing means of said temperature sensor housing. 15.Humidification apparatus as claimed in claim 2 wherein gas flow ischannelled within a conduit of known cross-sectional area, at least inthe region adjacent said flow probe means, and is provided with a sensorentry port adapted to receive said flow probe means, said sensor entryport being provided with a fixed locating depression and said flow probemeans being provided with a complimentary fixed locating tooth, thepositioning of said temperature and flow rate sensor housing meansrelative to said gas flow being controlled by the interconnection ofsaid locating depression and tooth.